The present invention relates to microchannel plate (MCP) technology. It finds particular application in conjunction with microchannel plates constructed of solid diamond grown using a chemical vapor deposition (CVD) process and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Microchannel plates are the electron producing section of photomultiplier tubes (xe2x80x9cPMTxe2x80x9d) used for night vision devices and general infrared (xe2x80x9cIRxe2x80x9d) detection equipment. Light enters the PMT through a photocathode, which converts the light signal into a very weak electronic signal. The electronic signal is amplified using an electron multiplier, which operates based on secondary electron emission. Secondary electron emission is the electronic property by which secondary electrons are produced as a result of primary electrons impinging on the material surface. Secondary electron coefficients (i.e., the ratio of secondary electrons produced from a single primary electron) range from two to four in most materials used for PMT applications, and as high as 25 for some exotic materials used for specialized PMT applications.
It is well known that chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) diamond, doped with specific impurities, is electrically conductive and produces copious amounts of secondary electrons. A cesiated diamond surface will produce more than 40 electrons at 1000 eV primary electron energy. Since 40 electrons are produced from each collision, and the process is multiplicative, a diamond MCP can produce millions of times more electrons than state-of-the-art MCPs. CVD Diamond growth processes, however, would likely be unable to coat prefabricated MCP structures since the xe2x80x9cporesxe2x80x9d or microchannels are typically about 6 xcexcm in diameter and about 360 xcexcm deep. Diamond deposition processes require background pressures, which would likely prevent source gasses from filling these pores.
A microchannel plate may be used as a secondary electron multiplier. Typically, the MCP consists of millions of glass microchannels in the form of capillary tubes, which are assembled and fused together to form a two-dimensional array in the shape of a disk. The capillary tubes are formed by drawing down glass-filled, glass-jacketed rods, arid then etching out the glass filling. Typical microchannel diameters range from about 40 xcexcm to about 10 xcexcm, with the corresponding channel pitch being such that the channel cross-sectional areas constitute about 50% of the total MCP face area. Metal films, deposited on both faces of the disk, serve as electrodes for applying an electric field across each channel and also electrically connect the multitude of channels together in parallel. Each channel behaves as a sort of continuous-dynode electron multiplier. The input end of an MCP-based detector includes a suitable photocathode optimized for the spectral characteristics of the incident radiation. The photocathode receives the incident photons and generates the primary photoelectrons, which then enter the glass capillary channels. The capillary material is specially chosen (the most common being a lead-oxide glass) such that when electrons impinge on the channel walls, secondary electrons are generated. These secondary electrons are accelerated by the voltage applied across the electrodes and travel in a parabolic trajectory along the length of the channels, until, due to the transverse component of their motion, they collide with the channel walls and dislodge additional secondary electrons with each impact, thus producing electron multiplication, or gain.
Detection of low-level signalsxe2x80x94optical (infrared, visible, ultraviolet and X-ray) as well as particle (electrons and ions)xe2x80x94is a critical requirement in a wide variety of applications, both military and civilian. A good example of devices whose performance de pends on their ability to amplify very low-level input signals with large gain (xe2x89xa7104) are night vision systems, which constitute an important part of the increasingly complex and technologically-intensive equipment employed in modem warfare. Currently available high-gain detectors include numerous types of photomultiplier tubes (PMTs) and image-intensifier tubes (IITs), many of which incorporate microchannel plates as the primary amplifying device.
Microchannel plates are almost always geometrically designed so that the channel axes are at a small angle (xe2x80x9cbias anglexe2x80x9d) to the perpendicular of the input and output faces.
Conventional MCP Manufacturing Technology
One process used in the industry for manufacturing microchannel plates is primarily based on the technology of drawing glass fibers and fiber bundles. The fabrication process begins with tubes of a specially formulated glass, usually a lead-oxide composition, that is optimized for secondary-electron emission characteristics. In the tubes are inserted soleid cores of a different glass with differential chemical etching characteristics. The filled tube is softened and drawn to form a monofiber. Millions of such fibers are now stacked together in a bundle in a hexagonal-close-packed format. The bundle is fused together at a temperature of about 500xc2x0 C. to 800xc2x0 C. and again drawn until the solid core diameters become approximately equal to the required channel diameter, which ranges from about 40 xcexcm down to  less than 10 xcexcm. Now individual microchannel plates are cut from this billet by slicing at the appropriate bias angle to the billet axis. The thickness of the slices is generally such that the channels have a length-to-diameter ratio of about 40-80.
The individual plates are ground and polished to an optical finish. The solid cores are removed by chemical etching in an etchant that does not attack the lead-oxide glass walls, thus producing the hollow channels. Further processing steps lead to the formation of a thin, slightly conductive layer beneath the electron-emissive surface of the channel walls. Electrodes, in the form of thin metal films, are then deposited on both faces of the finished wafer. Finally, a thin membrane of SiO2 (formed on a substrate which is subsequently removed) is deposited on the input face to serve as an ion barrier film, and the plate is secured in one of several different types of flanges. The finished MCP may now be incorporated in various image-intensifying detection systems.
For best understanding of the microchannel plate, it is best to review in detail the manufacturing process for conventional MCPs. FIGS. 1-3 illustrate the prior art MCP. FIGS. 4 and 5 show the conventional MCP production process of drawing and etching.
FIG. 1 shows an MCP. Microchannels 101 are held in a flange 102. Electrodes and connections, plus ion-barrier film and other items necessary for operation, are not shown. The function of the MCP is to augment the strength of the optical signal input without distortion.
FIG. 2 shows how electron amplification takes place. An electron 103 arrives from the left-to-right direction, and enters the microchannel tube 104. A voltage VD from a voltage source 105 supplies the high voltage necessary for the electron acceleration. Each electron, whether introduced as the electron 103 or generated within the tube 104, may dislodge one or more other electrons, which may proliferate to hundreds, or even hundreds of thousands, of electrons.
FIG. 3 shows how a photon striking electrode 106 dislodges an electron (exe2x88x92), which passes through chevron configuration microchannel plates 107, 108 with electron amplification and emerges as a great number of electrons (Gexe2x88x92), which strike a phosphor layer 109 and result in phosphorescent light output P.
FIG. 4 shows how a tiny glass-clad glass rod is drawn down to a capillary size, essentially by heating it to a softened state and then pulling. This leaves an inner glass 111 still clad in an outer glass 112, with both drawn down to a miniature capillary size. Additional steps are required to remove a glass core 111 to leave the hollow glass tube 112, drawn down to the microchannel size.
FIG. 5 shows how the glass-clad glass cores 111, 112 are bundled and drawn down in bundles to capillary size.
FIG. 6 shows how the drawn bundles of glass tubes are fused to form tubes 113 (which are still filled with glass) with fused walls 114. These bundles are placed at a small angle to the cutting saw and are sliced at places such as slice site 115 to provide microchannel blanks.
FIG. 7 shows schematically how a microchannel slice 115 has channels 116 at a slight angle to the perpendicular of the slice face. The microchannels 116 may be etched out with a suitable etchant, which attacks the core glass much more quickly than it might attack the cladding glass, leaving the tubes hollow.
FIG. 8 shows how the myriad of microchannels 116 are fused together and held in a flange 117.
FIG. 9 shows how ion-barrier film 118 and electrodes 119 are placed over the ends of the microchannels which are held together in a flange 117. Electrodes 119 are connected to a suitable voltage source. These electrodes are indium tin oxide or other transparent conductor. The ion-barrier film 118 may be SiO2.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.
A method for manufacturing a free-standing solid diamond microchannel plate produces a base structure, including a silicon material, having a backing and a vertical extension. A chemical vapor deposition diamond material is deposited onto the backing. The diamond material is substantially leveled with the vertical extension of the base structure. The silicon backing is removed from the leveled base structure.
In accordance with one aspect of the invention, the backing is removed by immersing the leveled structure into a solution including KOH.
In accordance with a more limited aspect of the invention, the base structure is produced so that at least one of the backing and the vertical extension include the silicon material.
In accordance with a more limited aspect of the invention, the vertical extensions are produced to have a diameter of about 6 xcexcm and a height of about 360 xcexcm.
In accordance with a more limited aspect of the invention, the diamond is deposited on the backing by filling a cavity, formed on the base structure by the vertical extensions, with the chemical vapor deposition diamond material.
In accordance with an even more limited aspect of the invention, the diamond material is leveled by machining excess diamond extending above the vertical extensions.
In accordance with an even more limited aspect of the invention, the chemical vapor deposition diamond material is polished to be substantially level with a top of the vertical extensions.
In accordance with another aspect of the invention, the chemical vapor deposition diamond material is polished with a laser machining process.
In accordance with another aspect of the invention, the base structure is produced by microfabricating the base structure using a deep reactive ion etch.
One advantage of the present invention is that a secondary electron coefficient greater than about 40 is achieved.
Another advantage of the present invention is that a significantly improved efficiency and output of night vision devices is achieved.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.